1. Field of the Invention
This invention relates to electrical devices comprising conductive polymers, and in particular to an improved means for making an electrical connection to a laminar electrode in such a device.
2. Summary of the Prior Art
Conductive polymer compositions [including such compositions which exhibit positive temperature coefficient (PTC), zero temperature coefficient (ZTC) or negative temperature coefficient (NTC) behavior] and electrical devices comprising them, are known. Reference may be made for example to U.S. Pat. Nos. 2,978,665 (Vernet et al.), 3,243,753 (Kohler), 3,311,862 (Rees), 3,351,882 (Kohler et al.), 4,017,715 (Whitney et al.), 4,085,286 (Horsma et al.), 4,095,044 (Horsma et al.), 4,177,376 (Horsma et al.) and 4,177,446 (Diaz) and to copending and commonly assigned applications Ser. Nos. 818,711 (Horsma et al.), 963,372 (Horsma), 965,343 (Van Konynenburg et al.), now U.S. Pat. No. 4,237,441, 965,344 (Middleman et al.), now U.S. Pat. No. 4,238,812, 969,928 (Van Konynenburg et al.), 6,773 (Simon), now U.S. Pat. No. 4,255,698, 38,218 (Middleman et al.), 41,107 (Walker), now U.S. Pat. No. 4,272,471, and the continuation-in-part thereof filed July 10, 1980, Ser. No. 167,364, 75,413 (Van Konynenburg), 84,352 (Horsma et al.), 85,679 (Toy et al.), 88,344 (Lutz) now abandoned, 98,711 (Middleman et al.), 98,712 (Middleman et al.), 102,576 (Brigham), the application filed by Brigham on Dec. 7, 1979, Ser. No. 102,621, now abandoned, 134,354 (Lutz), 141,984 (Gotcher et al.), 141,987 (Middleman et al.), 141,988 (Fouts et al.), 141,989 (Evans), 141,990 (Walty), 141,991 (Fouts et al.), 142,053 (Middleman et al.), 142,054 (Middleman et al.) and the application filed by Cardinal et al. on July 31, 1980, Ser. No. 174,136. The disclosure of each of these patents and patent applications is incorporated herein by reference. The term "conductive polymer" composition is used herein to denote a composition which has a resistivity of less than 10.sup.6 ohm.multidot.cm at a temperature between 0.degree. C. and 200.degree. C., preferably at 25.degree. C.
In many such devices, current is passed through the conductive polymer by means of laminar electrodes, and the electrical leads to the remainder of the circuit are attached to the electrodes. The electrodes are generally composed of a material having a resistivity of at most 5.times.10.sup.-2 ohm.multidot.cm, preferably a metal. The thickness of the electrodes may be such that there is a substantial potential drop from one end of the electrode to the other or it may be such that all points on any particular electrode are at substantially the same potential. When the device comprises a heat-recoverable member, the electrodes must deform easily when the member is heat-recovered and, if applied before the member is rendered heat-recoverable, in the process in which it is rendered heat-recoverable. When the devices are subject to temperature cycling, differences between the thermal coefficients of expansion of the electrode materials and the conductive polymers tend to result in separation of the electrode from the conductive polymer element. This is of course highly undesirable. The problem is particularly severe when the conductive polymer element comprises a PTC or NTC conductive polymer element, since the PTC or NTC effect depends upon a change in the volume of the PTC or NTC element. It is, therefore, preferred to use an electrode which can accommodate the amount of movement which is to be expected in preparation and/or use of the device, especially an electrode having a plurality of apertures therein e.g. a metal mesh or grid, the apertures of the electrode preferably being of a size such that the conductive polymer can penetrate into the apertures and anchor the electrode and the conductive polymer to each other. Unfortunately, however, there are serious problems in securing electrical leads to these preferred electrodes.